A lambda regulation in connection with a catalyzer is nowadays the most efficient exhaust gas purifying procedure for the Otto engine. Very low exhaust gas values can only be achieved in interaction with currently available ignition and injection systems. The nowadays-used catalyzers have the features to reduce hydrocarbons, carbon monoxide and nitrous gases up to more than 98% if the engine is operated in a range of 1% around the stoichiometric air-fuel relation with lambda=1. The lambda value indicates how much the actually present air-fuel mixture deviates from the mass relation of 14.7 kg air and 1 kg fuel that is theoretically required for a complete combustion. Lambda is hereby the quotient of the added air mass and the theoretical air demand.
The lambda probe is also used for diesel engines, for example in order to avoid emission scatter, which can occur for example due to component tolerances.
A lambda probe or wide-band lambda probe is preferably used as the sensor element for determining the concentration of the remaining oxygen in the exhaust gas. The Nernst cell of a lambda probe provides a voltage jump at an oxygen concentration that corresponds with the lambda value=1 and delivers thereby a signal, which shows whether the mixture is richer or leaner than lambda=1. The efficiency of the lambda probe is based on the principle of a galvanic oxygen concentration cell with a solid body electrolyte.
Being constructed as two-point probes the lambda probes work in an acquainted manner according to the Nernst principle based on a Nernst cell. The solid electrolyte consists of two boundaries that are separated by a ceramic. The used ceramic material becomes conductive at about 350° C., so that at different oxygen percentages on both sides of the ceramic, the so-called Nernst voltage is produced between the boundaries. This voltage is a measure for the relation of the oxygen partial pressures on both sides of the ceramic. Since the remaining oxygen content in the exhaust gas of a combustion engine strongly depends on the air-fuel relation of the mixture that is added to the engine, it is possible to use the oxygen content in the exhaust gas as a measure for the actually present air-fuel relation.
In order to monitor the ideal air-fuel mixture composition, wide-band lambda probes are preferably used in the exhaust gas system. These probes are typically operated at temperatures between T=750° C. and T=800° C.
If a rich mixture is present, the oxygen concentration in the exhaust gas lies below an oxygen concentration that is typical for a stoichiometrically running combustion, the lambda value is therefore <1 and produces a voltage >450 mV in the Nernst cell. If a lean mixture is present, the Nernst voltage falls below 450 mV. The lambda probe however only delivers reliable values if the probe, and especially the ceramic body of the probe, provide an operating temperature of approximately >400 C.
The described cascade voltage characteristic of the two-point probe only allows a regulation in a narrow value range around lambda=1. A significant extension of this measuring area is allowed by wide-band lambda probes, at which, in addition to the Nernst cell, a second electro chemical cell, known as a pump cell, is integrated. At the wide-band lambda probe, the exhaust gas diffuses into the pump cell, whereby oxygen is added to or withdrawn from the pump cell over a pump current until the pump cell provides an oxygen concentration that corresponds with a lambda=1. The required pump current is proportional to the oxygen partial pressure, which is actually present in the exhaust gas.
A procedure for operating a wide-band lambda probe is already known from DE 101 47 390 A1, at which the oxygen content of an exhaust gas is determined with the aid of a Nernst voltage with a reference voltage, whereby a pump cell is impinged with a pump current in the case of deviations from a lambda value=1. The pump current is a measure for the value of lambda in the exhaust gas. When activating a cold probe it is provided that the Nernst voltage is kept close to the reference voltage with the aid of a pre-controlling until the Nernst voltage becomes an actual measure for the oxygen concentration in the cavity of the pump cell.
Further, it is known that the determination of a gas concentration in a measuring gas is influenced by the pressure of the measuring gas. The functioning of the gas probe requires that an inflow of the measuring gas is specifically set in a measuring room over a diffusion barrier. The inflow of the measuring gas is basically subject to the Knudsen diffusion. This means that the mean free path of the gas molecules is basically determined by the geometry of the diffusion barrier, typically the dimensions of the opening of the measuring cell. The inflow of the measuring gas is also influenced by the gas phase diffusion.
The mentioned diffusions are influenced by pressure changes of the measuring gas so that the pressure has to be considered for a precise concentration determination in the measuring gas. The pressure dependency of the concentration determination can be shown, for example, over a sensor specific compensation parameter, known as a k-value, as follows:
                                          O            ⁢                                                  ⁢            2            ⁢            _raw            ⁢                          (              p_exh              )                                            O            ⁢                                                  ⁢            2            ⁢            _raw            ⁢                          (                              p_                ⁢                0                            )                                      =                              p_exh                          k              +              p_exh                                ·                                    k              +                              p_                ⁢                0                                                    p_              ⁢              0                                                          formula        ⁢                                  ⁢        1            p—0 reference gas pressurep_exh exhaust gas pressureO2—raw(p—0) gas concentration raw signal at reference gas pressureO2—raw(p_exh) gas concentration raw signal at exhaust gas pressurek compensation parameter
The compensation parameter depends on the specific characteristics of a sensor and varies solely because of manufacturing scatterings. Furthermore the compensation parameter gradually changes also due to ageing effects.
For correcting the concentration measurement, the determined compensation parameter is deposited in an analysis set-up at the manufacturing or application of the gas sensor and considered at the determination of the gas concentration.